1. Field of the Invention
The present invention relates to a system for controlling the position of a transducer head in a disc memory unit, typically a so-called fixed disc device and more particularly to a system for controlling the position of a head in a disc memory unit in which, in response to a reference information signal recorded on a recording medium disc, the position of the head is detected and controlled.
2. Description of the Prior Art
A conventional disc memory unit is provided with one or more discs as a medium for recording information and the disc or discs are normally rotated at speeds as high as a few thousand revolutions per minute (r.p.m). On both the main surfaces of the disc tens or hundreds of concentric tracks are formed for recording information. One head is provided for each surface of the disc in order to write and read information into and from the surface of the disc. In the case of writing or reading, the head is brought to a proper position in relation to a specific track selected from a pluarlity of tracks. In principle, the head is positioned so that its center line is coincident with the center line of the specific track.
As the technology of data storage has been advanced, many efforts have been made to reduce the dimensions of a disc and to increase the information storage density to an amount which would be as high as possible.
When data storage density is relatively low, as in the past, an open loop control system can be employed to control the position of the head. In this open loop control system only a pulse instruction, in the form of a predetermined number of pulses is applied to an actuator (such as a stepping motor) so as to properly position the head in relation to a selected track with a specified track designation number, but no detection is made at all for detecting the present position of the head. As a result, fine adjustment of the position of the head is not performed.
It is reported that in the case of a fixed disc of 5.25 inches in diameter, the open loop control system is limited to 500 TPI (track/inch) at the most.
When the data storage density is increased further and exceeds 700 TPI, a closed loop control system must be used to determine the position of the head. In this case, the position of the head must be detected with a high degree of accuracy. To this end, various methods and devices have been proposed and in most cases reference information or so-called servo information for the detection of the position of the head is recorded in advance on a disc. The servo information is read out by the head itself so that the readout reference information is processed to detect the present position of the head.
A closed loop control system based upon the above-described underlying principle is disclosed in detail in, for instance, Japanese Laid-open Patent Application No. 50-99709.
The prior art devised various patterns of reference information recorded on a disc and various methods for processing the readout reference information, but in general a circuit for processing detected reference information is quite complicated. Furthermore, since each pattern of reference information has its own meaning, the reference information must be recorded on a disc with a high degree of accuracy.
When such closed loop control systems including the control system of the type described above were first introduced, they were based upon the technical principle that while desired information is written into and read out from a disc, the position of the head is always controlled. For that purpose, reference information used for detecting the position of the head must be provided in parallel with recording regions in which desired information is stored. As a result, the prior art closed loop control system exhibits a contradiction in that at least one side or surface of a disc must be reserved for recording reference information.
However, it has been known in the art that when a disc rotates at a high speed, such parallel control as described above is not necessarily needed and that when the position of the head is controlled once for each rotation of the disc, the head can be maintained at a predetermined position with a sufficient degree of accuracy. As a result, the area for recording reference information of a disc can be decreased accordingly.
The prior art system will be described in more detail with reference to FIGS. 1-4.
FIG. 1 is a side view of a fixed disc memory unit in which a plurality of discs 1 are fixed as a magnetic recording medium to a spindle 1a which in turn is rotated at a high speed by a spindle motor 1b. Each surface of the disc 1 has a concentric area which defines a plurality of tracks 2 as shown in FIG. 2 and each track 2 has a sector area which is inserted into the track 2 and into which reference information 3 is stored.
Heads 4 are arranged to be opposite to the respective surfaces of the discs 1 to write and read information into and from the respective surfaces of the discs 1. The heads 4 are mounted to head arms 4a and then carried by a carrier 5 which is movable in the directions indicated by an arrow P. The carrier 5 is guided by guides 5a and is driven to be maintained at a controlled position by means of an actuator 6 through a thin metal band 5b.
FIG. 3 shows a pattern of the prior art reference information 3. Reference information 3a and 3b for the center track 2b of the three tracks 2a, 2b and 2c is shown. Each reference information 3a or 3b consists of alternately arranged S and N poles from the left to the right in FIG. 3. The recording patterns 3a and 3b are so defined that when the disc 1 is displaced in the direction indicated by an arrow Q, the N poles change to the S poles simultaneously at the midpoint m between the leading edge l and the trailing edge n, but on both sides of the midpoint m, the change from the N pole to the S pole does not occur simultaneously. A plurality of patterns each consisting of S and N poles as shown in FIG. 3 are repeated in the actual recording patterns 3a and 3b.
FIG. 4 illustrates waveforms of the signals derived from the head 4 when the head 4 reads out the reference information 3a and 3b. For the sake of easy understanding, it is assumed that the head 4 is moved upwardly from the normal position P0 for the track 2b and is located at the position P1 which is spaced apart from the position P0 by a distance equal to one half of the pitch between two adjacent tracks. Then, the position of the head 4 is in line with the position of reference information 3a so that, as shown in the case of the head position P1 of FIG. 4, the readout signal has not only a peak Va correspondong to the reference information 3a but also a peak Vm corresponding to the midpoint m between the ends of the reference information 3a. These peaks Va and Vm have opposite polarities as shown.
On the other hand, when the head 4 is moved downward from the normal position P0 and is located at the position P2 which is spaced apart from the normal position P0 by a distance equal to one half of the pitch between two adjacent tracks, the readout signal has a peak Vb corresonding to the reference information 3b and a peak Vm corresponding to the midpoint between the ends of the reference information 3b.
Therefore, it is apparent that when the head 4 is at the normal position P0, not only the peaks V and Vb corresponding to the reference information 3a and 3b but also the peak Vm corresponding to the midpoint between the ends of the reference information 3a and 3b appear at the same time. In this case, values of the peaks Va and Vb are equal to each other and, as is readily imagined, are substantially equal to one half of a value of the sole peak Va or Vb which appear when the head 4 is displaced by one half pitch upward or downward as described above.
However, the value of the center peak Vm having the opposite polarity remains unchanged regardless of the position of the head 4. Furthermore, the time that the peak Va appears and the time that the peak Vb appears are always separated from each other by Wt corresponding to the difference in the S- and N-pole patterns between the reference information 3a and 3b.
The two peaks Va and Vb are obtained from the waveform of the readout signal independently of each other with the center peak Vm being used as a reference. Furthermore, the deviation of the head 4 from its normal position P0 can be detected from the difference between the values of the two peaks Va and Vb. That is, when the value of the peak Va is higher than that of the peak Vb, the head 4 is deviated upwardly from its normal position P0 toward the position P1. On the other hand, when the value of the peak Va is lower than that of the peak Vb, the head is deviated downwardly from its normal position P0 to the position P2.
When detecting the position of the head or a deviation of the head from its normal position by the prior art system of the type described above, it is necessary to detect the values of the peaks which appear in the signal obtained when the reference information is read out or an analog signal representative of the difference between the values of the two peaks with a very high degree of accuracy. The value of each peak must be temporarily stored as an analog signal before the value is detected by a peak hold ciruit. This means that, in order to minimize an error which results from the above-described process, a high-accuracy circuit is required.
As described above, according to the prior art system, a circuit for separating the peaks which appear in the output signal derived when the reference information is read out, a circuit for holding the value of each of the peaks thus separated from each other and a circuit for comparing the values of the separated peaks must be upgraded or become quite complicated. In addition, in order to detect the position of the head with a high degree of accuracy, the reference information must be recorded with a high degree of accuracy, so that means for recording the reference information on a disc also becomes complicated and expensive. Furthermore, a relatively large area which is sufficient to record such reference information must be secured on the surface of a disc.